Consumer optical disks having a recording density of about 100 GB can now be commercialized due to Blu-ray Disc (registered trademark) standards using blue-violet semiconductor laser. In the future, optical disks are expected to have a capacity larger than 500 GB. However, to implement the optical disks having such a super high density, a novel-type densification technique is required which is different from a conventional densification technique involving shorter wavelength and higher NA of an objective lens.
For example, a technique for eliminating reflected light from the surface of a disk is disclosed in JP-A-2010-2575 (Patent Literature 1). According to JP-A-2010-2575, “an angle, which is formed between the direction of reflected light 15 that is incident light 11 reflected on a surface 3f of a hologram recording medium 3 and the direction of an optical axis of reproduction signal light 14, is larger than a diverging angle θ of the reproduction signal light 14. Accordingly, since the reflected light 15 is shielded by a pinhole filter 5 and a lens barrel 8, the reflected light 15 is not made incident on a light detector 7 as stray light. As a result, the quality of reproduction signals can be improved”.